Semiconductor device and method for fabricating the same

ABSTRACT

A semiconductor device is disclosed. The semiconductor device includes: a substrate; a gate structure disposed on the substrate, wherein the gate structure has a high-k dielectric layer; a first seal layer disposed on a sidewall of the gate structure, wherein the first seal layer is an oxygen-free seal layer and is non-L-shaped; and a second seal layer disposed on a sidewall of the first seal layer, wherein the second seal layer is an L-shaped seal layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.13/052,115, filed on Mar. 21, 2011, and all benefits of such earlierapplication are hereby claimed for this new continuation application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor device, and more particularly,to a semiconductor device with metal gate and method for fabricating thesame.

2. Description of the Prior Art

With a trend towards scaling down size of the semiconductor device,conventional methods, which are used to achieve optimization, such asreducing thickness of the gate dielectric layer, for example thethickness of silicon dioxide layer, have faced problems such as leakagecurrent due to tunneling effect. In order to keep progression to nextgeneration, high-K materials are used to replace the conventionalsilicon oxide to be the gate dielectric layer because it decreasesphysical limit thickness effectively, reduces leakage current, andobtains equivalent capacitor in an identical equivalent oxide thickness(EOT).

On the other hand, the conventional polysilicon gate also has facedproblems such as inferior performance due to boron penetration andunavoidable depletion effect which increases equivalent thickness of thegate dielectric layer, reduces gate capacitance, and worsens a drivingforce of the devices. Thus work function metals are developed to replacethe conventional polysilicon gate to be the control electrode thatcompetent to the high-K gate dielectric layer.

However, there is always a continuing need in the semiconductorprocessing art to develop semiconductor device renders superiorperformance and reliability even though the conventional silicon dioxideor silicon oxynitride gate dielectric layer is replaced by the high-Kgate dielectric layer and the conventional polysilicon gate is replacedby the metal gate.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a semiconductordevice with metal gate and method for fabricating the same.

According to a preferred embodiment of the present invention, asemiconductor device is disclosed. The semiconductor device includes: asubstrate; a gate structure disposed on the substrate, wherein the gatestructure comprises a high-k dielectric layer; and a first seal layerdisposed on a sidewall of the gate structure, wherein the first seallayer is an oxygen-free seal layer.

According to another aspect of the present invention, a method forfabricating semiconductor device is disclosed. The method includes thesteps of: providing a substrate; forming a gate structure on thesubstrate, wherein the gate structure comprises a high-k dielectriclayer; forming a first seal layer on a sidewall of the gate structure;and forming a lightly doped drain in the substrate adjacent to two sidesof the gate structure.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 illustrate a method for fabricating a semiconductor devicehaving metal gate.

FIGS. 7-12 illustrate a method for fabricating a semiconductor devicehaving metal gate according to another embodiment of the presentinvention.

FIG. 13 illustrates a semiconductor device having metal gate accordingto an embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-6, FIGS. 1-6 illustrate a method for fabricating asemiconductor device having metal gate, in which the method preferablyconducts a gate-first approach accompanying a high-k first fabrication.As shown in FIG. 1, a substrate 100, such as a silicon substrate or asilicon-in-insulator (SOI) substrate is provided. A plurality of shallowtrench isolations (STI) 102 used for electrical isolation is also formedin the substrate 100.

Next, a gate insulating layer 104 composed of oxide or nitride is formedon the surface of the substrate 100, in which the gate insulating layer104 is preferably used as an interfacial layer. Next, a stacked filmcomposed of a high-k dielectric layer 106, a polysilicon layer 108, anda hard mask 110 is formed on the gate insulating layer 104. Thepolysilicon layer 108 is preferably used as a sacrificial layer, whichcould be composed of undoped polysilicon, polysilicon having n+ dopants,or amorphous polysilicon material.

The high-k dielectric layer 106 could be a single-layer or a multi-layerstructure containing metal oxide layer such as rare earth metal oxide,in which the dielectric constant of the high-k dielectric layer 106 issubstantially greater than 20. For example, the high-k dielectric layer106 could be selected from a group consisting of hafnium oxide (HfO₂),hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON),aluminum oxide (AlO), lanthanum oxide (La₂O₃), lanthanum aluminum oxide(LaAlO), tantalum oxide, Ta₂O₃, zirconium oxide (ZrO₂), zirconiumsilicon oxide (ZrSiO), hafnium zirconium oxide (HfZrO), strontiumbismuth tantalite (SrBi₂Ta₂O₉, SBT), lead zirconate titanate(PbZr_(x)Ti_(1-x)O₃, PZT), and barium strontium titanate(Ba_(x)Sr_(1-x)TiO₃, BST). The hard mask 110 could be composed of SiO₂,SiN, SiC, or SiON.

Next, as shown in FIG. 2, a patterned photoresist (not shown) is formedon the hard mask 110, and a pattern transfer is performed by using thepatterned photoresist as mask to partially remove the hard mask 110, thepolysilicon layer 108, the high-k dielectric layer 106, and the gateinsulating layer 104 through single or multiple etching processes. Afterstripping the patterned photoresist, a gate structure 112 is formed onthe substrate 100.

Next, a first seal layer 114 composed of silicon nitride is formed onthe sidewall surface of the gate structure 112 and the surface of thesubstrate 100, and a lightly doped ion implantation is carried out toimplant n-type or p-type dopants into the substrate 100 adjacent to twosides of the gate structure 112 for forming a lightly doped drain 116.

As shown in FIG. 3, a second seal layer 118 composed of silicon oxideand a third seal layer 120 composed of silicon nitride are sequentiallyformed on the substrate 100 and covering the gate structure 112 and thefirst seal layer 114. In this embodiment, the second seal layer 118 ispreferably composed of silicon oxide and thus having a different etchingrate with respect to the first seal layer 114 underneath.

Next, as shown in FIG. 4, a dry etching process is performed topartially remove the third seal layer 120 and stop on the surface of thesecond seal layer 118, another dry etching is carried out to partiallyremove the second seal layer 118 and the first seal layer 114, and a wetetching process is performed to remove remaining polymers from the aboveetching process for forming a first spacer 122 composed of L-shapedfirst seal layer, an L-shaped second seal layer 118, and a second spacer124 composed of the remaining third seal layer 120 on the sidewall ofthe gate structure 112.

In an alternative approach to the above steps, another embodiment of thepresent invention could also perform a dry etching process to partiallyremove the third seal layer 120 and stop on the surface of the secondseal layer 118, perform another dry etching process to partially removethe third seal layer 118, and perform a wet etching process to partiallyremove the first seal layer 114 for forming the above L-shaped firstspacer 122, the L-shaped second seal layer 118, and the second spacer124.

Next, an ion implantation process is performed to implant n-type orp-type dopants into the substrate 100 adjacent to two sides of theaforementioned spacer for forming a source/drain region 126. In thisembodiment, a selective strain scheme (SSS) can be used for forming thesource/drain region 126. For example, a selective epitaxial growth (SEG)can be used to form the source/drain region 126, such that when thesource/drain region 126 is a p-type source/drain, epitaxial siliconlayers with silicon germanium (SiGe) can be used to form the p-typesource/drain region 126, whereas when the source/drain region 126 is ann-type source/drain region 126, epitaxial silicon layers with siliconcarbide (SiC) can be used to form the n-type source/drain region 126.Additionally, silicides (not shown) are formed on the surface of thesource/drain region 126. Thereafter, a contact etch stop layer (CESL)128 and an inter-layer dielectric (ILD) 130 layer are sequentiallyformed on the substrate 100. Since the steps of forming the abovementioned elements are well-known to those skilled in the art, thedetails of which are omitted herein for the sake of brevity.

As shown in FIG. 5, a planarizing process, such as a chemical mechanicalpolishing (CMP) is conducted to partially remove the ILD layer 130, theCESL 128, and the patterned hard mask 110 until exposing the polysiliconlayer 108. Another adequate etching process could then be carried toremove the polysilicon layer 108 to form a trench 132. During this step,the high-k dielectric layer 106 could be used as an etching stop layerto protect the gate insulating layer 104 underneath from the etchingprocess conducted previously. As the aforementioned planarizing processand etching process are well known to those skilled in the art, thedetails of which are omitted herein for the sake of brevity.

Next, as shown in FIG. 6, a work function metal layer 134, a barrierlayer 136, and a low resistance metal layer 138 are formed sequentiallyto fill the trench 132, in which the work functional metal layer 134could include a p-type work function metal or an n-type work functionalmetal. A planarizing process is conducted thereafter to partially removethe low resistance metal layer 138, the barrier layer 136, and workfunction metal layer 134 for completing the fabrication of asemiconductor device having metal gate 140.

Referring to FIGS. 7-12, FIGS. 7-12 illustrate a method for fabricatinga semiconductor device having metal gate according to another embodimentof the present invention, in which this embodiment also employs agate-first fabrication with a high-k first process.

As shown in FIG. 7, a substrate 200, such as a silicon substrate or asilicon-in-insulator (SOI) substrate is provided. A plurality of shallowtrench isolations (STI) 202 used for electrical isolation is also formedin the substrate 200.

Next, a gate insulating layer 204 composed of oxide or nitride is formedon the surface of the substrate 200, in which the gate insulating layer204 is preferably used as an interfacial layer. Next, a stacked filmcomposed of a high-k dielectric layer 206, a polysilicon layer 208, anda hard mask 210 is formed on the gate insulating layer 204. Thepolysilicon layer 208 is preferably used as a sacrificial layer, whichcould be composed of undoped polysilicon, polysilicon having n+ dopants,or amorphous polysilicon material.

Next, as shown in FIG. 8, a patterned photoresist (not shown) is formedon the hard mask 210, and a pattern transfer is performed by using thepatterned photoresist as mask to partially remove the hard mask 210, thepolysilicon layer 208, the high-k dielectric layer 206, and the gateinsulating layer 204 through single or multiple etching processes. Afterstripping the patterned photoresist, a gate structure 212 is formed onthe substrate 200.

Next, a first seal layer (not shown) composed of silicon nitride isformed on the sidewall surface of the gate structure 212 and the surfaceof the substrate 200, and an etching back process performed to partiallyremove the first seal layer on the substrate 200 for forming a firstspacer 214 on the sidewall of the gate structure 212. Next, a lightlydoped ion implantation is carried out to implant n-type or p-typedopants into the substrate 200 adjacent to two sides of the gatestructure 212 for forming a lightly doped drain 216. A second seal layer218 composed of silicon oxide is then covered on the gate structure 212,the first spacer 214, and the surface of the substrate 200.

As shown in FIG. 9, a third seal layer 220 composed of silicon nitrideis formed on the substrate 200 and covering the gate structure 212 andthe second seal layer 218. In this embodiment, the second seal layer 218is preferably composed of silicon oxide and thus having a differentetching rate with respect to the third seal layer 220 above.

As shown in FIG. 10, a dry etching process is performed to partiallyremove the third seal layer 220 and stop on the surface of the secondseal layer 218, and a wet etching process is performed to partiallyremove the second seal layer 218 for forming a first spacer 214, anL-shaped second seal layer 218, and a second spacer 222 on the sidewallof the gate structure 212.

Next, an ion implantation process is performed to implant n-type orp-type dopants into the substrate 200 adjacent to two sides of theaforementioned spacer for forming a source/drain region 226. In thisembodiment, a selective strain scheme (SSS) can be employed for formingthe source/drain region 226. For example, a selective epitaxial growth(SEG) can be used to form the source/drain region 226, such that whenthe source/drain region 226 is a p-type source/drain, epitaxial siliconlayers with silicon germanium (SiGe) can be used to form the p-typesource/drain region 226, whereas when the source/drain region 226 is ann-type source/drain region 226, epitaxial silicon layers with siliconcarbide (SiC) can be used to form the n-type source/drain region 226.Additionally, silicides (not shown) are formed on the surface of thesource/drain region 226. Thereafter, a contact etch stop layer (CESL)228 and an inter-layer dielectric (ILD) 230 layer are sequentiallyformed on the substrate 200. Since the steps of forming the abovementioned elements are well-known to those skilled in the art, thedetails of which are omitted herein for the sake of brevity.

As shown in FIG. 11, a planarizing process, such as a chemicalmechanical polishing (CMP) is conducted to partially remove the ILDlayer 230, the CESL 228, and the hard mask 210 until exposing thepolysilicon layer 208. Another adequate etching process could then becarried to remove the polysilicon layer 208 to form a trench 232. Inthis step, the high-k dielectric layer 206 could be served as an etchingstop layer to protect the gate insulating layer 204 underneath from theetching process conducted previously. As the aforementioned planarizingprocess and etching process are well known to those skilled in the art,the details of which are omitted herein for the sake of brevity.

Next, as shown in FIG. 12, a work function metal layer 234, a barrierlayer 236, and a low resistance metal layer 238 are formed sequentiallyto fill the trench 232, in which the work functional metal layer 234could include a p-type work function metal or an n-type work functionalmetal. A planarizing process is conducted thereafter to partially removethe low resistance metal layer 238, the barrier layer 236, and workfunction metal layer 234 for completing the fabrication of asemiconductor device having metal gate 240.

Overall, the present invention preferably forms an oxygen-free seallayer on the sidewall of the gate structure to protect the high-kdielectric layer in the gate structure before a lightly doped drain isformed. According to a preferred embodiment of the present invention,the oxygen-free seal layer is preferably composed of silicon nitride,and is adhered and contacting the hard mask, the polysilicon layer, thehigh-k dielectric layer, and gate insulating layer of the gatestructure. As no material layer is formed on the sidewall of the gatestructure for protecting the high-k dielectric layer before theformation of lightly doped drain in conventional art, the high-kdielectric layer is often damaged or removed during later processesincluding the wet cleaning conducted for lightly doped drain, oxidestripping, or spacer removal. By forming an oxygen-free seal layer onthe sidewall of the gate structure before forming the lightly dopeddrain, the present invention could avoid the aforementioned problemfound in conventional art and prevent the high-k dielectric layer fromdamage effectively.

It should be noted that despite the aforementioned embodiment employs agate-first and h-k first approach, the fabrication process of thepresent invention could also be applied to gate-first fabrication andhigh-k last fabrication, which are all within the scope of the presentinvention. For instance, the gate structure of the gate-first processpreferably includes a gate insulating layer, a high-k dielectric layerdisposed on the gate insulating layer and a polysilicon gate disposed onthe high-k dielectric layer, in which the high-k dielectric layerpreferably to be a linear high-k dielectric layer. The gate structure ofa high-k last fabrication on the other hand, as shown in FIG. 13,includes a gate insulating layer 204, a high-k dielectric layer 206disposed on the gate insulating layer 204, and a metal gate 240 disposedon the high-k dielectric layer 206, in which the high-k dielectric layer206 is a U-shaped high-k dielectric layer.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a gate structure disposed on the substrate, wherein the gate structurecomprises a high-k dielectric layer; a first seal layer disposed on asidewall of the gate structure, wherein the first seal layer is anoxygen-free seal layer and is non-L-shaped; and a second seal layerdisposed on a sidewall of the first seal layer, wherein the second seallayer is an L-shaped seal layer.
 2. The semiconductor device of claim 1,wherein the first seal layer is a first spacer.
 3. The semiconductordevice of claim 2, wherein the second seal layer is disposed on asidewall of the first spacer and the second seal layer comprises amaterial different from the first seal layer.
 4. The semiconductordevice of claim 3, wherein the etching rate of the second seal layer isdifferent from the etching rate of the first seal layer.
 5. Thesemiconductor device of claim 1, further comprising a second spacerdisposed on the second seal layer.
 6. The semiconductor device of claim1, wherein the gate structure comprises: a gate insulating layer; thehigh-k dielectric layer disposed on the gate insulating layer; and apolysilicon gate disposed on the high-k dielectric layer.
 7. Thesemiconductor device of claim 6, wherein the high-k dielectric layercomprises a linear high-k dielectric layer.
 8. The semiconductor deviceof claim 1, wherein the gate structure comprises: a gate insulatinglayer; the high-k dielectric layer disposed on the gate insulatinglayer; and a metal gate disposed on the high-k dielectric layer.
 9. Thesemiconductor device of claim 8, wherein the high-k dielectric layercomprises a linear high-k dielectric layer or a U-shaped high-kdielectric layer.
 10. A method for fabricating semiconductor device,comprising: providing a substrate; forming a gate structure on thesubstrate; forming a first seal layer on a sidewall of the gatestructure, wherein the first seal layer is an oxygen-free seal layer andnon-L-shaped; forming a lightly doped drain in the substrate adjacent totwo sides of the gate structure; and forming a L-shaped second seallayer on a sidewall of the first spacer, wherein the L-shaped secondseal layer comprises a material different from the first seal layer. 11.The method for fabricating semiconductor device of claim 10, furthercomprising: performing a first etching process to partially remove thefirst seal layer before forming the lightly doped drain for forming theremaining first seal layer into a first spacer on the sidewall of thegate structure; forming a second seal layer on the gate structure, thefirst spacer, and the substrate; forming a third seal layer on thesecond seal layer; performing a second etching process to partiallyremove the third seal layer for forming a second spacer; and performinga third etching process to partially remove the second seal layer forforming the L-shaped second seal layer on a sidewall of the firstspacer.
 12. The method for fabricating semiconductor device of claim 11,wherein the first seal layer comprises silicon nitride, the second seallayer comprises silicon oxide, and the third seal layer comprisessilicon nitride.
 13. The method for fabricating semiconductor device ofclaim 11, wherein the first etching process comprises an etching backprocess, the second etching process comprises a dry etching process, andthe third etching process comprises a wet etching process.
 14. Themethod for fabricating semiconductor device of claim 10, wherein thegate structure comprises: a gate insulating layer; a high-k dielectriclayer disposed on the gate insulating layer; and a polysilicon gatedisposed on the high-k dielectric layer.
 15. The method for fabricatingsemiconductor device of claim 14, wherein the high-k dielectric layercomprises a linear high-k dielectric layer.
 16. The method forfabricating semiconductor device of claim 10, wherein the gate structurecomprises: a gate insulating layer; the high-k dielectric layer disposedon the gate insulating layer; and a metal gate disposed on the high-kdielectric layer.
 17. The method for fabricating semiconductor device ofclaim 16, wherein the high-k dielectric layer comprises a linear high-kdielectric layer or a U-shaped high-k dielectric layer.